Near-infrared magnetic core-shell nanoparticles based on lanthanide metal-organic frameworks as a ratiometric felodipine sensing platform

Felodipine is an effective drug to treat hypertension, but its abuse can cause bardycardia. It is significant to develop highly sensitive detection platform for felodipine to enable the efficient treatment of hypertension diseases. In this work, to highly efficiently detect felodipine, multi-emission near-infrared (NIR) hierarchical magnetic core-shell lanthanide-MOF nanoparticles, namely Nd-MOF@Yb-MOF@SiO2@Fe3O4 (NIR-1), has been synthesized by layer-by-layer (LBL) method. LBL method can adjust the optical properties of NIR-1 and expose more active sites to improve sensitivity in detection process. NIR-1 has near-infrared luminescence emission, which can efficiently avoid the interference of autofluorescence in biological tissues. Photo-luminescent (PL) experiments also reveal that NIR-1 could be used as a near-infrared ratiometric luminescent sensor for felodipine detection with high selectivity and sensitivity, the low of detection limit (LOD) is 6.39 nM in felodipine detection, which is also performed using real biological samples. In addition, NIR-1 can be used as a ratiometric thermometer could also be applied in the temperature sensing from 293 K to 343 K. Finally, detection mechanisms for felodipine and temperature sensing performance based on near-infrared (NIR) emission were also investigated and discussed in detail.

With continuous development of the biological field, traditional MOF-based sensors are also facing urgency challenges such as biological imaging and biosensing. Near-infrared (NIR) luminescent detection has become a rising star because of its ability to cope with these challenges. In 2013, Foucault-Collet et al. successfully designed and synthesized NIR nano Ln-MOF, Yb-PVDC-3. When Yb-PVDC-3 was phagocytosed and internalized, NIR luminescent imaging of living hela cells was realized, and the Yb emission signal can remained stable for 13 h 14 . NIR luminescent sensors have many advantages over visible-light MOFs sensors, for example, large Stokes shift of NIR luminescent can avoid the background fluorescence interference of biological fluids. Additionally it can reduce the photodamage to biological tissue samples. NIR luminescent material has higher vital penetration ability and can better penetrate biological tissues for imaging.
Single emission of Ln-MOFs has poor anti-interference ability, which is easy to be affected by external environment and the ratiometric luminescence probes can deal with this problem well 15,16 . In 2015, Zhang et al. successfully synthesized multiemission Ln-MOFs, MZMOF-3. It has a self-correcting function and has been successfully used to detect the biomarker for ovarian cancer in serum samples 17 . In 2020, Gomez et al. successfully synthesized NIR multi-emission Ln-MOFs, [LnCl(NDC) (DMF)] (Ln 3+ = Yb 3+ , Nd 3+ ), it has excellent temperature sensing performance in the range of 15-300 K 18 . Ratiometric luminescent probe can resist external environment interference and NIR luminescent probe can avoid autologous fluorescence interference of biological samples. Therefore, considering the above advantages, it is meaningful to construct NIR ratiomeric luminescent probe for biological sample detection and temperature sensing. By detecting optical signals of these NIR, composition analysis of internal environment of organisms can be realized. Non-contact temperature sensing protects biological samples, and nano-scale temperature sensing also has the possibility of application.
Multi-shell MOF@magnetic core nanoparticles synthesized by layer-by-layer (LBL) method have many unique advantages. Firtly, nanoparticles prepared based on spherical magnetic core are also spherical, and it have a larger specific surface area compared with blocky MOFs, which could improve the sensitivity of detection; Secondly, the size of nanoparticles is adjustable, which affects the optical properties of nanoparticles; Thirdly, multi-component MOFs make it possible to customize pore shapes and anisotropic frameworks, enriching the geometry of nanoparticles. Therefore, the Ln-MOFs sensors prepared by this strategy have great application potential 19 . For example 20 , Shi's and coworkers synthesized magnetic core/multishell Ln-MOF Fe 2 O 3 @SiO 2 @Eu-MOF@Tb-MOF (MagMOF) nanoballs by layer-by-layer synthesis 20 . MagMOF has been successfully used to detect acute myocardial infarction (AMI) biomarkers with high sensitivity, including creatine kinase isoenzyme (CK-MB), troponin I (CTn I), and myoglobin (Mb).
Hypertension is a significant cause of death and disease worldwide, which significantly increases the risk of heart, brain, and kidney disease 21 . According to the World Health Organization (WHO), in the past 30 years, the number of people with high blood pressure worldwide has doubled to 1.4 billion 22 . 0.7 billion patients did not diagnosis timely and suffered from it. The initial treatment of hypertension can effectively control by felodipine, but its abuse can cause excessive peripheral vasodilation with marked hypotension and bardycardia 23 . In order to monitor the concentration of felodipine in patients' serum, it is the urgent need for developing a novel method to realize anti-hypertensive drugs sensing, which have better therapeutic effect on hypertension. In 2022, Han et al. constructed a ratiometric luminescent sensor by using carbon dots and gold nanoclusters to achieve efficient antihypertensive drug captopril detection 24 . Furthermore, the probe was prepared into a luminescent test strip, and the visual sensing for quantitative detection of captopril was successfully realized with the assistance of a smartphone, the LOD is 101.3 nM. However, to the best of our knowledge, for the detection example for antihypertensive drug felodipine detection still remains a challenge.
In this work, considering the advantages of the ratiometric luminescent probe and NIR lanthanide luminescent sensing, we successfully synthesized multi-emission NIR Yb/Nd-MOF by layer-by-layer method ( Fig. 1), namely Nd-MOF@Yb-MOF@-SiO 2 @Fe 3 O 4 (NIR-1). We apply Fe 3 O 4 @SiO 2 as template to form nanoball NIR-1 as the surface of Fe 3 O 4 @SiO 2 attached with carboxylate group can coordinated with Ln 3+ ions further to construct MOF layer. Compared with the previous work (MagMOF) 20 , NIR-1 has two innovations. Firstly, the NIR emission probes can be better applied to the biological sample detection. Secondly, a series of NIR-1 nanoparticles with different properties were synthesized and tested by adjusting the layer of NIR-1. Photoluminescence (PL) experiment showed that NIR-1 could be used as the first near-infrared ratiometric luminescent probe for felodipine with high selectivity and sensitivity, the detection sensitivity parameter is calculated to be 1.4 × 10 7 [M] -1 and LOD is 6.39 nM. Additionally, NIR-1 also can be applied as a ratiometric luminescent thermometer for temperature sensing. Further, the blind box experiment proved that NIR-1 could judge the content of mixed drugs (felodipine, bisoprolol and carvedilol) in the solutions through the analysis of luminescence signal response. Finally, the detection mechanisms for felodipine and temperature sensing performance were investigated and discussed in detail.
Results and discussion PXRD patterns, FT-IR spectra, UV-vis spectrum, TEM and fluorescent spectrum of NIR-1. Powder X-ray Diffraction (PXRD) patterns of Yb-MOF, Nd-MOF and NIR-1 have been characterized, all of them possess the similar structural motifs, which also confirm the purity of NIR-1 (Fig. 2a). Additionally PXRD intensity of SiO 2 @Fe 3 O 4 as the magnetic core of NIR-1 is very weak, which is consistent with previous reported work (Fig. 2b) 20 . Single-crystal structure of Yb-MOF and Nd-MOF were described in detail in a previous report (Supplementary Fig. 1 and Supplementary Fig. 2 Supplementary Fig. 1a). In the Nd-MOF, one L 3coordinated with three Yb atoms (Nd1), which have the same coordination environment. Nd1 is coordinated by seven O atoms (O1, O2, O5, O6, O7, O8, O9) from carboxyl groups of L 3and two O atoms (O3, O4) from DMF ( Supplementary Fig. 2a).
In the Ultraviolet-visible (UV-vis) spectrum, NIR-1 has two absorption peaks, and the maximum absorption occurs at 330 nm, which can be attributed to the H 3 L ligand ( Supplementary Fig. 3a). Felodipine has two absorption peaks, located at 255 nm and 355 nm ( Supplementary Fig. 3b). In the Fourier Transform Infrared (FT-IR) spectra ( Supplementary Fig. 4a), the stretching vibration peak of the C = O bond in NIR-1 is 1660 cm -1 , which is shifted to lower wavenumber compare with the stretching vibration peak of the C = O bond in the pure ligand (1688 cm -1 ), demonstrating the carboxyl groups are coordinated with Yb 3+ and Nd 3+26-34 . Fluorescent properties of NIR-1 in the DMF solution are also investigated ( Supplementary Fig. 4b). NIR-1 has three NIR characteristic emission peaks at 978 nm ( 2 F 5/2 → 2 F 7/2 ) origin from Yb 3+ and 1056 nm ( 4 F 3/2 → 4 I 11/2 ) origin from Nd 3+ , respectively. The excitation spectrum of NIR-1 indicated that 330 nm excitation light source is the most suitable experimental conditions ( Supplementary Fig. 5). The quantum yield of NIR-1 is 0.07%. We think that the low quantum yield are mainly influenced by the following factors: Firstly, NIR-1 has a layered structure, the main contribution to quantum yield comes from Yb/Nd ions emissions in the outermost layer and most of the Yb/Nd ions are in the interior of NIR-1; Secondly, we only tested the quantum yield from 950 to 1100 nm (980 nm for Yb and 1056 nm for Nd) and did not include the ligand emission.
In Transmission Electron Microscope (TEM) images, the mapping images showed the multilayer structure of NIR-1 ( Fig. 3) [35][36][37][38][39][40] . The magnetic core is inside, which consist of magnetic iron oxide and silicon dioxide. The outer shell layer structure is composed of Yb-MOF and Nd-MOF respectively ( Fig. 3c-e). Yb-MOF and Nd-MOF are uniformly distributed on the surface of the magnetic core. Elements content of Yb and Nd is 0.31% and 0.08%, respectively (Supplementary Table 1). The thickness of the MOF layer can be controlled the by layer-bylayer method. Figure 4 shows that the outer layer thickness of NIR-1 increases as the number of layer-by-layer methods increases (9.86 nm to 21.63 nm). As the pure Fe 3 O 4 @SiO 2 magnetic core are only coated with SiO 2 , the thickness of outer layer is <6.5 nm (Fig. 4e).
Variable temperature solid-state fluorescent spectrum of NIR-1. In this work, solid-state photo-luminescence spectroscopy of NIR-1 in the temperature range of 293 K to 353 K has been observed ( Fig. 5a). With the temperature increase, the intensities  of both emission bands gradually increase, the luminescence intensity of Yb 3+ increases much more obviously than that of Nd 3+ . Based on the variation difference between these two emissions, the two emissions located at 980 nm and 1058 nm can be selected as ratiometric temperature sensors, which can be applied as self-calibrating temperature measurements. To simplify the description, the ratio of dual emission at 980 nm and 1058 nm represents Δ. On this basis, the temperature sensing performance of NIR-1 was quantitatively described. It is a cure with an increasing trend and shows a good nonlinear relationship between Δ and temperature (Fig. 5b, c). The nonlinear formula is fitted to equation 1: Δ = 3.1511*10 -6 e 0.0378T + 0.9966, and correction coefficient is 0.9983. S r (relative thermal sensitivity) can be calculated by the equation |∂Δ/∂T | /Δ. The definition of |∂Δ/∂T | / Δ is signal to noise ratio of the luminescent spectrometer which is 30000:1. According to the nonlinear equation 1, the evolution of S r under different temperature was presented, the maximum S r for NIR-1 is 2.17% K -1 at 343 K. To verify the stability of the NIR-1, we carried out repeated experiments. Through the cyclable experiments ( Supplementary Fig. 10), NIR-1 can be reusable at least 3 times for temperature sensing which reflects the stability of NIR-1.
Host-guest energy transfer process is affected by temperature change, which leads to the increase of emission intensity. To confirm the energy transfer direction, solid-state NIR-1 has three characteristic emission bands at 890 nm, 980 nm and 1058 nm, which represent the emission of Nd 3+ ( 4 F 3/2 → 4 I 9/2 ), Yb 3+ ( 2 F 5/2 → 2 F 7/2 ) and Nd ( 4 F 3/2 → 4 I 11/2 ) (Fig. 5a), which means that the energy transfer direction should be from the Nd 3+ to Yb 3+ in NIR-1. Additionally, there is overlap between emission spectrum of L 3and excitation spectrum of Yb 3+ / Nd 3+ ( Supplementary Fig. 5), which represents to there is a strong energy transfer between ligand and lanthanide ions. The reason why enhancement of temperature with the increase emission of NIR-1 based on a thermally driven phonon-assisted transfer mechanism. The similar situation can be found in previous work 41 , when the temperature increases from 90 K to 240 K, the lifetime of Tb-BABDC-PBMA ( 5 D 4 ) decreases by 49.5%, while the lifetime of Eu 0.0025 Tb 0.9975 -BABDC-PBMA ( 5 D 4 ) decreases by 81.3%. This result indicates that the energy transfer efficiency of Tb 3+ to Eu 3+ is stably enhanced at high temperature. Similarly, the energy transfer efficiency of Nd 3+ to Yb 3+ is also stably enhanced at high temperature.
To explore the relationship between energy transfer efficiency and temperature, the solid luminescent lifetime of NIR-1 and Nd-MOF at 1058 nm was investigated, respectively. With the temperature increasing (293 K to 353 K), the lifetime of NIR-1 increased from 2.67 μs to 4.91 μs, while the lifetime of Nd-MOF inreased from 2.95 μs to 7.28 μs (Figs. 5d and 5e). Obviously, the presence of energy transfer acceptor Yb 3+ in NIR-1 leads to the  difference of Nd 3+ emission lifetime parametera between NIR-1 and Nd-MOF. The efficiency of energy transfer between the donor (Nd 3+ ) and acceptor (Yb 3+ ) can be calculated from the lifetime of donor luminescence: E = 1-τ da /τ d , where τ da and τ d are the donor's excited-state lifetime in the presence and absence of the acceptor, respectively. As shown in Fig. 5f, the energy transfer efficiency from Nd 3+ to Yb 3+ is gradually increases with the temperature increasing, which proved that the phonon-assisted Foster transfer mechanism [42][43][44][45][46][47][48][49] .
Compared with other existing dual emission MOFs-based thermometers 50-56 , S r value of NIR-1 is higher ( Table 1). The results indicated that NIR-1 has the excellent fluorescent temperature sensing performance in the near-infrared emission region.
Ratiometric luminescent sensor for felodipine by NIR-1. Insufficient use of hypertension drugs can cause a rebound in blood pressure, while overdose can cause liver damage, hypotension, and even drug poisoning 57 . Therefore, it is very significant to establish highly sensitive and efficient methods for qualitative and quantitative detection of hypertension drugs. In this work, we pioneered the detection of felodipine by using NIR-1.
NIR-1 is a near-infrared luminescent probe used to sense various hypertension drugs. We prepared five drugs for the treatment of hypertension (felodipine, nifedipine, bisprolol, carvedilol, and amlodipine besylate) to test the luminescent sensing properties of NIR-1. The emission peaks located at 978 nm and 1056 nm were quenched to different degrees with the addition of drugs (40 nM). It is worth noting that the quenching amount of luminescent intensity caused by the addition of felodipine is the largest compared with other medications. The quenching efficiency at the same concentration is 4-10 times that of other drugs. Obviously, this difference in quenching efficiency reveals that felodipine can be selectively identified among many antihypertensive drugs (Fig. 6).
To further explore the potential of NIR-1 in felodipine quantitative detection, the suspension of NIR-1 was titrated with felodipine. The emission peaks at 978 nm and 1056 nm were gradually quenched with the addition of the felodipine (from 0-40 nM) (Fig. 7a). To explore the relationship between the concentration of felodipine and luminescent intensity ratio of NIR-1, which are represented by I 978 nm /I 1056 nm and [C] (concentration of felodipine). I 978 nm /I 1056 nm and [C] has a good linear relationship using the following equation: I 978 nm /I 1056 nm = -14107 [C] + 3.47 (R 2 = 0.99), the quenching effect (K sv ) is calculated to be 1.4 × 10 7 [M] -1 , and low of detection (LOD) is 6.39 nM (Fig. 7b).
In addition, NIR-1 in prepared using different synthesis steps (7 cycle steps and 21 cycle steps) to conduct felodipine detection experiments. For the NIR-1 material obtained by 7 cycle steps, the relationship between the concentration of felodipine   (Supplementary Fig. 7). Both K sv values of NIR-1 nanoballs synthesized by different cycle steps are smaller than the K sv of NIR-1 obtained by 14 cycle steps. This indicated that NIR-1 obtained by 14 cycle steps has the best performance in felodipine detection, which may attributed to NIR-1 obtained by 14 cycles possess Ln 3+ strong emission and appropriate number of defect sites to attract felodipine further to realize quenching effect. To highlight the advantages and stability of NIR-1, the parallel detection experiment of felodipine based on Yb-MOF and Nd-MOF is also performed. For Yb-MOF ( Supplementary Fig. 8 Anti-interference performance of NIR-1 in the felodipine detection process. The antihypertensive drugs used in this work are divided into two types: Calcium channel antagonists (felodipine, nifedipine, and amlodipine besylate) and beta-blocker (bisoprolol and carvedilol). Doctors usually combine different antihypertensive drugs for patients with severe hypertension to achieve better treatment effects. Therefore, in felodipine's detection process, other medicines often interfere. Consequently, it is necessary to conduct selective testing of mixed drugs for NIR-1. We have prepared four additional samples: A mixture of felodipine and carvedilol, a mixture of felodipine and bisoprolol, a mixture of carvedilol and bisoprolol, a mixture of felodipine, bisoprolol, and carvedilol. Each drug in the solutions was a concentration of 2 μM. Suspension of NIR-1 is titrated by using mixed durg solutions. It can be concluded that felodipine plays a leading role in the quenching effect to NIR-1 emission, the contribution of bisoprolol and carvedilol to luminescent quenching is not apparent, but with the addition drug types, the quenching efficiency also slightly increased (1.4 × 10 7 [M] -1 to 1.6 × 10 7 [M] -1 ) ( Fig. 7b and Supplementary Fig. 11). This makes it is possible to judge the sample contains additional drugs by the value of quenching efficiency. To further explore the NIR-1's quantitative detection ability of felodipine in the presence of interference, the suspension of NIR-1 was titrated with the  mixture of calcium channel antagonists drugs ( Supplementary  Fig. 11a). The relationship between concentration of the mixture and I 978 nm /I 1056 nm is calculated as follows: I 978 nm /I 1056 nm = -15980[C] + 3.36 (R 2 = 0.99), the K sv is calculated to be 1.6 × 10 7 [M] -1 (Supplementary Fig. 11b) 53 . This reflects the NIR-1 has the excellent anti-interference ability in the sensing experiment.
Blind box experiment for the analysis of background drug interference. NIR-1 not only has excellent anti-interference performance, but also has the ability to analyze background interference (judge the sample contains additional drugs by the value of quenching efficiency). For the five drug solutions with known concentration, the quenching efficiency increased slightly with the increase number of drugs ( Supplementary Fig. 12a). Then, we conducted a blind box experiment, firstly, unlabled A is containing two solutions: felodipine solution, a mixed solution containing felodipine, bisoprolol, and carvedilol. Each drug in the solutions was the same concentration. The type and concentration of the solutions in the sample B. Sample B contain two solutions and further detected by NIR-1. The experimental results show that the two solutions behave differents quenching efficiency to NIR-1. The solution with higher quenching efficiency contained three drug components than pure felodipine solution towards NIR-1, it is consistent with the previous experimental results (Supplementary Fig. 12b).
Detection application of NIR-1 for felodipine in the real biological samples. Further, the detection application of NIR-1 for felodipine in the real serum samples is also performed. The maximum blood concentration(C max ) is an important index to evaluate the effect of drug therapy, when the connection of felodipine is higher than C max , patients will be suffer the danger even damage their life., To help patients obtain the better therapeutic effect, the development of facile and sensitive felodipine detection platform in the range of C max is of great significance. Therefore, NIR-1's luminescent performance towards felodipine is performed in the human serum. Based on previous studies, the C max of felodipine is 12 nM [58][59][60][61] . Felodipine solution (in human serum) is used to titrate NIR-1. The recovery rate can be found between 98.69%-102.53% and the RSD is <1.14% (Table 2). This demonstrates the NIR-1 has excellent performance to detect felodipine in human serum.
Detection mechanism of felodipine based on NIR-1 sensing platform. To explore the luminescent quenching mechanism for felodipine based on NIR-1 sensing platform, PXRD and luminescent lifetime of NIR-1 in the detection process is investigated. Firstly, PXRD result showed that NIR-1 could keep its structural stability in the detection process of felodipine, luminescent quenching of NIR-1 is not caused by structural collapse 62 . Secondly, luminescent lifetime parameters are significant for analyzing the detection mechanism. The lifetime of NIR-1 solution (0.2 g/L) with the addition of felodipine is investigated. The lifetime of L 3at the position of 370 nm has been decreased from 0.825 ns to 0.788 ns with the addition of felodipine in different concentration (from 0 to 500 nM) (Fig. 8a) (Table 3). Given to there are some aromatic rings in felodipine so it can attach with H 3 L through the π-π stacking and oxygen atom and "-NH" group in felodipine can easily combine with H 3 L in NIR-1 through hydrogen bonding interaction, these excited-state hydrogen bond/ π-π stacking interactions strengthening facilitates internal conversion, which lead to felodipine can interrupt the energy transfer progress from H 3 L to Ln 3+ ions 63 . The NIR lifetime of Yb-MOF at the position of 978 nm has been reduced from 11.31 μs to 6.77 μs (Fig. 8b) (Table 4) and the NIR lifetime of Nd-MOF at the position of 1056 nm has been reduced from 6.66 μs to 5.22 μs with the addition of felodipine in different concentration (from 0-200 nM) (Fig. 8c) (Table 5), which means that felodipine can further lead to energy transfer process between Yb 3+ and Nd 3+ , which leads to the emission quenching of the Yb 3+ / Nd 3+64, 65 . As a result, the quenching effect of Yb 3+ /Nd 3+ emissions bands were mainly caused by the hydrogen bond/π-π stacking interactions and interruption of energy transfer between Yb 3+ and Nd 3+ is caused by felodipine 66,67 .

Discussion
In this work, multi-emission ratiometric luminescent probe NIR-1 was synthesized by the LBL deposition method, which has been characterized by PXRD, FT-IR, UV-vis spectrum, TEM and luminescent spectrum. NIR-1 achieves temperature sensing in the range of 293 K to 353 K based on the emission intensity ratio of Yb 3+ (978 nm) and Nd 3+ (1056 nm). In addition, NIR-1 also can be used as a ratiometric luminescent sensor for felodipine detection with high efficiency and sensitivity. We believe that NIR hierarchical Ln-MOF has great potential application in drugs detection, the detection of C max in the human serum sample should help patients to avoid drug abuse. NIR-materials probes can avoid many external interference such as the emission located between 400-600 nm from organism, As a result, it can be expected that NIR hierarchical Lanthanide-MOF magnetic coreshell nanoparticles would have great potential applications to other molecular drugs' detection in real body fluid.
Step 1: 5 mL Ytterbium(III) acetate solution (8 mM, DMF and H 2 O as solvent) was added to the beaker. The mixture was heated to 353 K and stirred for 5 min. After washing and centrifuging (8000 r/min, 3 min), the product was transferred to a beaker, and the supernatant was removed.
Step 2: Step 2 was the same as that of step 1 just usage 5 mL H 3 L ligand suspension (5 mM, DMF and H 2 O as solvent) not Ytterbium(III) acetate solution.
Step 3: Step 3 was the same as that of step 1 just usage 5 mL Neodymium(III) trichloride solution   Step 4: Same as step 2.
Step 1-4 forms a complete cycle of steps, the above synthetic routes can repeat the cycle for 14 times. The white powdered NIR-1 was obtained after washed with DMF and ethanol three times, respectively.

Preparation of different analytical solutions
Preparation of Yb-MOF suspension. Yb-MOF (2 mg) and DMF (10 mL) were added to a 25 mL beaker, then the mixture was under an ultrasonic bath 30 min to prepare the 0.2 g/L Yb-MOF suspensions.
Preparation of Nd-MOF suspension. Nd-MOF (2 mg) and DMF (10 mL) were added to a 25 mL beaker, then the mixture was under an ultrasonic bath 30 min to prepare the 0.2 g/L Nd-MOF suspensions.
Preparation of different drug solutions. The following drugs such as felodipine, Nifedipine, Bisoprolol, Carvedilol, Amlodipine Besylate were added to 10 mL DMSO under an ultrasonic bath for 5 min to prepare the 2 μM drug solutions for future use.
Preparation of biological samples. Felodipine (3.8 mg) were added to 10 mL DMSO under an ultrasonic bath for 5 min to prepare the 1 mM felodipine solutions. Then, dilute the felodipine solution with serum to 2 μM for future use.